There are two parts to this problem.
One is to reduce the back emf from the coil that generates the
voltage to arc across the opening contacts. A diode across the
coil will achieve this.
Two is to maintain a near zero voltage across the contacts as
they open, for long enough to open far enough to prevent an arc
forming. This is solved with an RC snubber across the contacts.
36 Orr Crescent
ph +64 (0)4 569 1963
On 7.01.21 11:33 pm, Ian Richardson via
In the pipe organ
world, the use of diodes in direct electric actions is standard
practice. I have just fitted up a direct action unit extension
organ with about 200 electromagnetic valves, each runs on 15
vdc. and carries a current of about 250mA. Each coil is shunted
with 1N400X diodes or equivalent. There is no sparking - imagine
if there was, the organ woiuld break down every day as the coils
are activated hundreds of times with each performance! The
diodes cost about €1 per 100.
The point about polarity is, of course, valid but simply
solved by using a high current silicon diode in series with
the power supply. If connected the correct way round, all
works well; if the wrong way round, it doesn't work at all, so
it "fails safe".
From: John Hubert <jfphubert@...>
Sent: Thu, 7 Jan 2021 11:17
Subject: Re: [synchronomeelectricclock] Master Clock contact
I do just as you suggest on all my dials and masters.
1N400X series diodes. The maintaining of current is
minimal (looked some years ago with an oscilloscope and
it was insignificant). There is typically some
variation of impulse length dependant on the master
anyway (Gents tend to be longer and early Synchronome
clocks rather short - lighter parts?). Typically, the
coil carries about 300 mA and has a resistance of about
5 Ohms, with a parallel resistor of about 50 Ohms.
Diodes work well - but of
course you have to have the right polarity.
Surely now we have
cheap reliable silicon diodes the best approach
is the now-standard practice of connecting a
diode across the coil so that when the contact
opens the diode shorts out the inductive kick?
The diode should be reverse biased when the coil
is energised. The only snag with this is that
the diode maintains the current when the contact
opens for a period which may cause a problem.
That can be made shorter by putting a resistor
in series with the diode to dissipate the stored
energy more quickly at the expense of a higher
voltage. For example, if the coil carried 1 amp
when the contact was closed, and the resistor
was 100 ohms, the peak backswing voltage would
be 100 volts, quite a lot but probably not
enough to cause breakdown in the gap.